Add cancel control for task polling

[HarperZ9]

Summary

Adds a cancel control for tool calls that are running as MCP tasks and currently being polled from the Tools tab.

Root cause

The app already exposed tasks/cancel through the Tasks tab, but a user who launched a tool as a task from the Tools tab only saw the disabled Polling Task... button. The active polling task ID was not stored in a way that Tools could cancel, and the polling loop could continue into tasks/get after a user wanted to stop it.

Changes

• Track the active polling task ID/run in App.
• Pass pollingTaskId and cancelTaskPolling into ToolsTab.
• Render a scoped Cancel Task button while a tool task is actively polling.
• Wake and stop the local polling delay after successful cancellation.
• Guard against late tasks/get / tasks/result responses overwriting the cancelled state.
• Add regression coverage for direct cancellation and an in-flight polling response race.

Closes #1039.

Validation

• npm.cmd test -- --runTestsByPath src/__tests__/App.taskPolling.test.tsx --runInBand
• npm.cmd run build-client
• npm.cmd run lint
• npx.cmd prettier --check client/src/App.tsx client/src/components/ToolsTab.tsx client/src/__tests__/App.taskPolling.test.tsx
• git diff --check

[cliffhall]

Closing: v1 is deprecated.

Thank you for this contribution, and apologies for the long wait for a response.

v1 will receive security fixes only. We reviewed every open v1 PR for security impact before closing — see the backlog triage in #1819 — and a small number were retained for a final 1.0.5 patch release. This one is a functionality, compatibility, or cleanup change rather than a vulnerability fix, so it is being closed unmerged. This is not a judgment on the quality of the work — it's a consequence of the branch it targets being frozen.

If the underlying problem still exists in v2, we'd genuinely like to know. Please open an issue describing it against v2. Note that we accept external contributions as issues rather than pull requests — maintainers handle design and implementation through a prompt-driven workflow. See CONTRIBUTORS.md.

Thanks again for taking the time to contribute to the Inspector.